Field of the Invention
The invention concerns a method for the adaptation of values of scan parameters and system parameters in a scan protocol for a magnetic resonance scan sequence on the basis of a limit value. The invention also concerns a magnetic resonance apparatus that is operable to implement such a method, as well as a non-transitory, computer-readable data storage medium encoded with programming instructions for implementing such a method.
Description of the Prior Art
Magnetic resonance (MR) imaging is a technology for generating MR images depicting an object under examination. MR imaging can achieve a high soft-tissue contrast. The object under examination, for example a patient, is typically positioned in a basic magnetic field that is static and as homogeneous as possible, with a field strength of between 0.5 tesla and 5 tesla, for example. The basic magnetic field aligns the magnetization of the nuclear spins of the object under examination; in particular, a polarization of the nuclear spin magnetization takes place along the direction of the basic magnetic field.
During an MR scan sequence, radio-frequency (RF) pulses are radiated in order to deflect the nuclear magnetization out of its rest position along the direction of the basic magnetic field, i.e. in order to excite the nuclear magnetization. The subsequent relaxation of the nuclear magnetization generates RF signals, so-called echoes. During gradient-echo MR imaging or echo-planar MR imaging (EPI), gradient echoes are generated selectively by using gradient pulses to rephase and dephase the nuclear magnetization. Refocusing RF pulses are used in spin-echo MR imaging.
Gradient pulses can be used for the spatial encoding of the MR data during the MR scan sequence. The gradient pulses generate gradient magnetic fields (gradient fields), which are superimposed on the basic magnetic field.
The MR data can be scanned (acquired) during a read-out phase of the MR scan sequence. The acquired MR data are also called raw data. The raw MR data are processed in order to reconstruct the MR image, composed of image data, of the object under examination. For example, the scanned MR data are typically digitized and are initially stored in a memory organized in the spatial-frequency space (k-space). It is then possible to use a Fourier transformation to transform the MR data into the image space in order to generate the MR image data.
The different parameters in an MR scan sequence are typically combined in a scan protocol. The scan protocol can be used in order to perform a specific MR scan sequence again later, for example with a new patient.
The scan protocol contains scan parameters and system parameters. The scan parameters determine properties of the MR scan sequence. The system parameters determine the underlying operation of hardware components of the MR system. At least the system parameters are defined in the scan protocol typically with reference to a reference coordinate system. Depending on the patient to be examined, it may then be necessary to transform at least some values of the system parameters from the reference coordinate system, for example a machine coordinate system of the MR system, into a patient coordinate system.
The scan protocol with the scan parameters and the system parameters is often determined within the context of scan planning. During scan planning, it is possible to determine values for the scan parameters and the system parameters by means of correlated planning. Scan planning can typically take a certain amount of time and/or require qualified operators. Therefore, techniques are known with which scan planning is performed in a planning phase, for example without any specific reference to the patient to be examined. The scan protocol obtained in this way can then be included in a scan phase for different patients and adapted as appropriate. In such case, then no replanning of the scan protocol is necessary during the scan phase. Typically, only the values of the system parameters are adapted to the specific patient coordinate system.
In the case of a coordinate transformation of this kind, the values of at least the system parameters can be significantly changed. As a result, technical and/or physiological limit values may be exceeded.
In order to avoid limit values being exceeded as a result of the adaptation of a scan protocol to the specific patient coordinate system, according to reference implementations the corresponding parameters of the MR scan sequence are frequently defined conservatively in the scan protocol, i.e. with a certain safety margin with respect to the corresponding limit values. For example, the amplitudes of gradient pulses in the scan protocol are frequently selected as no higher than 1/√3*G_Max, wherein G_Max designates a technological limit value of the maximum amplitude of the gradient pulses. In this case, a coordinate transformation into the patient coordinate system has sufficient headroom in order to enable a change in the values of the corresponding parameters within the prespecified limit values.
However, a technique of this kind has certain drawbacks. Typically, a technique of this kind requires the different hardware components of the MR system to be designed with respect to high limit values, wherein, due to the above-mentioned safety margin, the technical capacity of the hardware components is not always fully utilized in normal operation. In certain circumstances, this can increase the cost of the production of the MR system, although, for some applications, the increased technical capacity of the hardware components is not directly reflected in improved image quality.